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Limit maximum sigma to avoid overflows

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Limit the maximum sigma to avoid overflowing the blur calculation,
and more importantly to limit the size of buffers.

I checked that this failed on my linux box, and this CL fixes
the problem.

BUG=chromium:768294

Change-Id: I7ed14acc47f546db9c00c78c148a898459852a9f
Reviewed-on: https://skia-review.googlesource.com/50920
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
This commit is contained in:
Herb Derby 2017-09-26 11:10:20 -04:00 committed by Skia Commit-Bot
parent cd87140111
commit 9b89bcae40
1 changed files with 14 additions and 2 deletions
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16
src/core/SkMaskBlurFilter.cpp
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@ -526,9 +526,21 @@ static PlanningInterface* make_plan(SkArenaAlloc* alloc, double sigma) {
return plan;
};
// NB 136 is the largest sigma that will not cause a buffer full of 255 mask values to overflow
// using the Gauss filter. It also limits the size of buffers used hold intermediate values.
// Explanation of maximums:
// sum0 = window * 255
// sum1 = window * sum0 -> window * window * 255
// sum2 = window * sum1 -> window * window * window * 255 -> window^3 * 255
//
// The value window^3 * 255 must fit in a uint32_t. So,
// window^3 < 2^32. window = 255.
//
// window = floor(sigma * 3 * sqrt(2 * kPi) / 4 + 0.5)
// For window <= 255, the largest value for sigma is 136.
SkMaskBlurFilter::SkMaskBlurFilter(double sigmaW, double sigmaH)
: fSigmaW{std::max(sigmaW, 0.0)}
, fSigmaH{std::max(sigmaH, 0.0)}
: fSigmaW{SkTPin(sigmaW, 0.0, 136.0)}
, fSigmaH{SkTPin(sigmaH, 0.0, 136.0)}
{
SkASSERT(sigmaW >= 0);
SkASSERT(sigmaH >= 0);